As is known in the art, it is generally desirable that radio frequency (RF) power amplifiers operate as efficiently as possible. One technique for improving the efficiency of RF power amplifiers is known as envelope tracking. Using envelope tracking, efficiency can be improved by causing the direct current (DC) power supply voltage of the power amplifier to track the envelope of the RF input signal of the amplifier. By having the power supply voltage of the power amplifier track the envelope of the RF input signal, the amplifier can be maintained at or near a saturation point during operation, which is typically the most efficient region of operation. Example implementations of envelope tracking are described in, for example, U.S. Pat. No. 6,788,151 to Shvarts et al. and U.S. Pat. No. 7,482,869 to Wilson.
One problem with envelope tracking is that relatively complex circuitry is typically needed to provide a desired power supply voltage that is modulated to the envelope of the RF input signal. In some cases, the envelope signal may be generated by detecting the RF input signal, processing the detected signal to extract an envelope of the signal, and subsequently providing the envelope signal to a power supply modulator to generate the voltage for the power amplifier. In this approach, the circuits all have to be synchronized with one another, which can be relatively difficult to achieve. Furthermore, this technique is typically performed with analog signals. Consequently, the circuits used to generate the supply voltage for the power amplifier may be sensitive to differences in component operating characteristics caused by, for example, manufacturing tolerances and/or differences in the operating environment (e.g., ambient temperature, etc.).
In conventional power amplifiers that use envelope tracking, the bandwidth of the envelope signal is typically limited. For example, in some recent systems, power amplifiers are only capable of tracking multi-tone signals within a bandwidth of 40 MHz or less. However, many modern applications require much higher modulation bandwidths. In some cases, modulation bandwidths may be required that are at or near the RF frequency. Conventional envelope tracking techniques are not capable of supporting such modulation bandwidths. That is, in conventional systems, isolation between envelope and RF signals in an envelope tracking amplifier is typically provided using low pass filters. Low pass filters are not effective when the modulation frequencies approach those of the RF.
In an electronic warfare (EW) environment, counter-measure systems must be prepared to transmit multi-tone signals anywhere in a broad frequency band. These frequency bands may be as wide as 2 GHz or more in some cases. Current envelope tracking amplifier technology is not able to meet this demand for two reasons: (1) envelope-modulated power-supply technology cannot provide high power at high modulation frequencies, and (2) isolation between the envelope and RF signal is difficult to achieve
Techniques and circuits are desirable that are capable of providing envelope tracking power amplification in systems and networks where high modulation bandwidths are possible. Techniques and circuits are also desirable that are capable of providing isolation between envelope signals and RF signals in an envelope tracking power amplifier, without the use of filters.